Histone deacetylases (HDACs) are a family of enzymes found in numerous organisms among which bacteria, fungi, plants, and animals. Such enzymes catalyze the removal of acetyl groups from ε-N-acetylated lysine residues of various protein substrates including histones, transcription factors, α-tubulin, and nuclear importers. Up to date eighteen HDAC isoforms have been characterized. They are classified in four different families with regard to their DNA sequence similarity and their biological role within the cells.
HDAC1, HDAC2, HDAC8 and HDAC3 are members of class-I. The first three isoforms are primarily found in the nucleus; meanwhile HDAC3 is also found in the cytoplasm or membrane-associated.
HDAC4, HDAC5, HDAC6, HDAC7, HDAC9 and HDAC10 form class-II. This class has been further divided in two sub-classes, class IIa (HDAC4, 5, 7 and 9) and class IIb (HDAC6 and 10). Class-II enzymes are expressed in a limited number of cell types and either shuttle between the nucleus and cytoplasm (i.e., class-IIa), or are mainly cytoplasmic (i.e., class-IIb) (Yang X. J., et al., Mol. Cell. Biol., 2005, 25, 2873).
Class-IV comprises only one member (HDAC11), meanwhile class-III, also called sirtuins, is composed of NAD+ dependent enzymes. The common feature of classes I, II and IV enzymes resides in their zinc dependent nature. HDAC inhibitors (HDACi) have been shown to be potent inducers of growth arrest, differentiation and apoptotic cell death of transformed cells in vitro and in vivo.
HDAC inhibition was also shown to lead to the reduction of inflammation in models of autoimmune and inflammatory diseases (Leoni F., et al., Proc. Natl. Acad. Sci., 2002, 99, 2995).
One of the first compounds to have been documented as HDACi was the well-known anti-epileptic valproic acid, which inhibits all isoforms of classes I, II and IV. Once recognized the important role of this family of enzymes in the development of cancer, many efforts directed to find potent HDACi were undertaken by numerous academic groups as well as by pharmaceutical companies.
Vorinostat, originally known as SAHA (suberoylanilide hydroxamic acid), was the first-in-class small molecule hydroxamate derivative HDACi to have been approved by the FDA in 2006 to treat a rare cancer, cutaneous T-cell lymphoma in patients who have received at least one prior systemic therapy (Grant S., et al., Nature Rev. Drug Discov., 2007, 6, 21). SAHA is a potent HDACi inhibiting classes I and II as the vast majority of HDACi currently in clinical trials (Paris M., et al., J. Med. Chem., 2008, 51, 1505).
Actually, according to their structures, the various families of inhibitors can be grouped, in four main groups:
a) short chain fatty acids (e.g., sodium butyrate, phenylbutyrate, pivanex (pivaloyloxymethyl butyrate, AN-9), and valproic acid);
b) hydroxamates (e.g., SAHA, belinostat (PXD101), panobinostat (LBH589), dacinostat (LAQ-824), and trichostatin);
c) cyclic derivatives (e.g., romidepsin or FK-228);
d) benzamide (e.g., entinostat (MS-275), mocetinostat (MGCD-0103) and acetyldinaline (CI-994)).
Some clinical trials involving combination therapies have been conducted, to assess the efficacy of broad spectrum HDACi in combination with standard chemotherapeutic agents, (e.g., docetaxel and vorinostat), in patients with advanced and relapsed lung, bladder, or prostate cancer (clinical trial NCT00565227). HDAC has been hypothesized as a potential target for the treatment of parasite infections (e.g., Plasmodium infection) about thirteen years ago. If most efforts from the scientific community have been dedicated to the identification of selective HDACi, there is still a large medical need for pan inhibitors since it has been demonstrated that the various cancer diseases do not involve the same HDAC isoforms. Moreover, the scientific community is also divided with regard the assessment of specific HDAC isoforms to specific cancers (Giannini G., et al., Future Medicinal Chemistry, 2012, 4, 11, 1439-1460). Indeed, HDAC1 is up-regulated in prostate cancer (Halkidou K., et al., Prostate, 2004, 59, 177) and gastric cancer (Choi J. H., et al., Jpn. J. Cancer Res., 2001, 92, 1300), HDAC2 is up-regulated in gastric cancer (Song J., et al., APMIS, 2005, 113, 264), HDAC3 is up-regulated in lung cancer (Bartling B., et al., Lung Cancer, 2005, 49, 145) and there is elevated expression of HDAC6 in oral squamous cell carcinoma (Sakuma T., et al., J. Oncol., 2006, 29, 117).
The involvement of HDAC in further diseases such as neurodegenerative diseases (Chuang D. M., et al., Trends in Neuroscience, 2009, 32, 11, 591; Sleiman S. F., et al., Expert Opin. Investig. Drugs, 2009, 18, 5, 573), cardiac hypertrophy (Hamamori Y., et al., J. Clin. Invest., 2003, 112, 6, 824) has also been documented. A recent review details diseases for which HDAC inhibition is recognized as a new approach (Dinarello C. A., et al., Mol. Med., 2011, 17, 333).
The bidendate hydroxamic acid moiety is recognized to be one of the best zinc binding-group, and a multitude of HDAC inhibitors bearing such moiety has been developed (Sampath-Kumar A., et al., Bioorg. Med. Chem. Lett., 2005, 15, 8, 1969). However, such functionality has also been associated with poor pharmacokinetic properties (Colletti S., et al., Bioorg. Med. Chem. Lett., 2001, 11, 107) as well as with sustained toxicity (Suzuki T., Cur. Med. Chem., 2005, 12, 24, 2867). Therefore, a lot of efforts has been devoted to the identification of new HDACi that could demonstrate high binding affinity toward the biological target as well as potent cellular activity. A review published lately (Bertrand P., Eur. J. Med. Chem., 2010, 45, 2095), assessed the binding affinity and biological properties of various non-hydroxamate based derivatives, among which thio adducts were disclosed, hypothesizing that the latter could potentially have an orientation within the active site of the protein completely different from the one of hydroxamate analogues.
If HDACi containing a straight thio-binding group have been studied no such derivatives have however entered clinical trial so far.
Bertrand P. also proposed that FK228 biological activity was due to the reduction of the disulfide bond to lead to the thio adduct 1, the latter being the active entity as depicted in scheme 1 underneath.

However, this mechanism of action of FK228 can be questioned in front of U.S. Ser. No. 12/845,658 which hypothesized another metabolite to be the active species. Unfortunately, since no biological activity of the various theorized metabolites was shown, no clear teaching could be gathered from this work.
The straight thio analogue of SAHA (scheme 2) has been synthesized (Suzuki T., et al., Bioorg. Med. Chem. Lett., 2004, 14, 3313) and both compounds demonstrated similar HDAC affinity.

Suzuki T., et al. further disclosed more potent thio-containing metal binding groups (MBG) HDACi bearing a sterically more hindered amide moiety such as biphenyl, benzofuran, indole or quinoline instead of the phenyl group of SAHA (Suzuki T., et al., J. Med. Chem., 2005, 48, 1019).
JP2007238452 disclosed derivatives of Formula 2, wherein the carbon atom in position α with respect of the carbonyl amide was substituted by a carbamate moiety (i.e., R2═CO2R).

Such derivatives have also been disclosed later reporting optimization of the CAP group (Suzuki T., et al., J. Med. Chem., 2006, 49, 4809) and/or the spacer (Itoh Y., et al., J. Med. Chem., 2007, 50, 5425). Interestingly, such derivatives were disclosed as HDAC6 selective. In particular, Itoh Y. disclosed derivatives bearing medium-sized amino substitutents in position α with respect of the carbonyl amide which have been found to be HDAC6 selective supposedly because of the absence of hydrophobic pocket to accept such groups in the other isoforms.
It is also generally recognized that thio derivatives are one Log unit less active than their hydroxamate counterpart (Wang D., et al., J. Org. Chem., 2007, 72, 5446). SAHA has shown beneficial effects in a model of focal cerebral ischemia (Faraco G., et al., Mol. Pharmacol., 2006, 70, 6, 1876).
Therefore, a great need still exists in providing new HDAC inhibitors presenting low-nanomolar binding affinity toward the HDAC proteins as well as potent cellular activity.